Adaptive radar clutter rejection

ABSTRACT

A control loop in a radar receiver is jointly responsive to range gated video and a reference potential to establish a filter control signal. A plurality of range gated filters are responsive to the filter control signal to adjust the frequency response in a manner such that clutter is rejected yet the maximum frequency bandwidth is dynamically provided for moving target detection for varying clutter conditions. Each range gated filter has a filter element which is switchable such that the effective electrical properties of the element vary in accordance with the switching to thereby effect a change in the frequency characteristics of the filter. The switching rate is much higher than the pulse repetitive frequency of the radar such that the switching rate does not interfere with signals being processed through the filter. In one embodiment, a variable duty cycle pulse generator having a fixed frequency is utilized to effect control over the frequency characteristics of a filter.

States Patent 1 Doggett, Jr. I

Primary Examiner-T. H. Tubbe sing Attarney-Mueller & Aichele [11]3,721,978 l lMarch 20, 1973 [57] ABSTRACT A control loop in a radarreceiver is jointly responsive to range gated video and. a referencepotential to establish a filter control signal. A plurality of rangegated filters are responsive to the filter control signal to adjust thefrequency response in a manner such that clutter is rejected yet themaximum frequency bandwidth is dynamically provided for moving targetdetection for varying clutter conditions. Each range gated filter has afilter element which is switchable such that the effective electricalproperties of the element vary in accordance with the switching tothereby effect a change in the frequency characteristics of the filter.The switching rate is much higher than the pulse repetitive frequency ofthe radar such that the switching rate does not interfere with signalsbeing processed through the filter. In one embodiment, a variabledutycycle pulse generator having a fixed frequency is utilized to effectcontrol over the frequency characteristics of a filter.

7 Claims, 10 Drawing Figures 32 3| MAIN TIMING PULSES RADAR TIMING RADARRANGE GATING sIGNALs CIRCU'TS TRANsMITTER 3 INCLUDING AND VIDEO 7 RANGEGATE REcEIvER RANGE GATED L 4Gv GENERATING FILTER cIRcuITs (FIG. 5) 3K37 344 34A RANGE v "GATED RANGE GATEo vlDEo FILTER L 34B 2: l l 0 II IMOVING J RANGE GATEo zif g FILTER g "G6 55 .t 5 |ADAPTIVE AVERAGl/VG 659 :C/RCU/T 70 VARIABLE 58 r E I DUTY CYCLE l PUYLSE I INTEGRAToR IGENERATOR l (FIG. 9)

l I REFERENCE I L l 'PATENTEDHARZ'OIBH SHEET 10F 3 CLUTTER REGION FOR o20 FREQUENCY 2| RADAR PRF FILTER 60 2 co TRoL '5" T4 75 76 33 X 84 9oSAMPLE HI Low wag; AND. PASS PASS HOLD FILTER FILTER 77 RA GE 47 GATINGA sIGNAL 3| 8O 78 ELECTRONIC FULL wAvE SWITCH 'NTEGRATOR DETECTOR 36RANGE GATED TYPICAL RANGE GATED FILTER VIDEO l2l Q:

4 20 I23 I58 GATE GATED VIDEO TO FIG. 9 A

+ ADAPTIVE RANGE GATE ENABLING PULSE GATE ---REI=ERENcE INVENTOR.

JOHN G. DOGGETT, JR.

2M, QM) KW ATTORNEYS PATENTEDHARZOIQYS 312L978 SHEET 2 UF 3 30 I 32 3MAIN TIMING PULSES\ RADAR TIMING RADAR RANGE GATING sIGNALs CIRCU'TS ITRANsMITTER I I 3 INcLuDING AND vIDEo 47 RANGE GATE REcEIvER i RANGEGATED GENERATING 33 FILTER cIRcuITs (FIG. 5). 7 A36 L37 344 34A 49 RANGEL4) -GATED RANGE GATED VIDEO FILTER l l 0L34B II I 54 I i i: l

34X MovING J .JAEIIILR FILTER V a I v 66 55 -|AOAP7'/VE AVERAGl/VG' 64 I5 :c/Rcu/r GATE VARIABLE DUTY CYCLE 59 l PULSE I INTEGRATDR GENERATOR II I 5? FI G 9) I GATE L: -REFE ENGE I RADAR MAXIMUM RANGE RANGE OF l ITEREsT MAIN TIMING PUILSES [P N r {-43 F44 ADAPTIVE RANGE GATE 1 YENABLING PULSE 67 38 I I RANGE QUANT'Z'NG llll|llHlllHlHlllllllllllllllllllllllIlllL PULSES I 48 l 11 1 i so 1 IR IRANGE BIN GATING PuLsEs' l n l I Q I 45 l i n FIG. 4 INVENTOR JOHN G.DOGGETT, JR.

WW QM v ATTORNEYS ADAPTIVE RADAR CLUTTER REJECTION BACKGROUND OF THEINVENTION This invention relates to radar systems and more particularlyto radar systems which adaptively reject clutter. v t v In vehicle bornescanning radar systems, the ground clutter return has a Doppler effectin accordance with the speed of the vehicle and the direction of theantenna beam with respect to the direction of movement of the vehicle.Many airborne radar antennae emit energy in the form of a beam having asmall angle of divergence causing a finite beam width. This angle is amatter of design choice. In some radar applications the angle ofdivergence may be large, i.e., the beam is quite wideor even may beomnidirectional. For purposes of simplifying discussion, this discussionis limited to a radar antenna having a small angle of beam divergence.Even so, the intercept of terrain by the radar antenna emitted energy issufflciently large that the Doppler effect frequency shift in energyreflected toward such radar antenna from the terrian varies across thebeam width as will be briefly discussed. This phenomenon is well known.

First assume the radar antenna is forward looking such that the centerof the beam lies along the velocity vector of the aircraft carrying theradar system. As a result of the finite width of the emitted beam, theradiation returning to-the antenna is reflected from terrain bothdirectly ahead of and to either side of the aircraft. The width of thebeam at the intercept of the beam and the terrain is a function of thedistance from the antenna to the terrain and the angle of divergence.Since the beam width is finite, the. relative velocity between theaircraft and a terrain intercept varies across the width of the beam.This geometric relationship is well known. As a result there is a spreadin the Doppler shift of frequency of the reflected energy. Since asingle antenna or antenna system receives all the reflected energywithin the beam width the terrain reflectedenergy (clutter) appearsacross a finite bandwidth of frequencles.

This variation or spread of Doppler shift in frequency of the clutter isbroadened when the antenna scans transverse to the direction of aircrafttravel and is pointing at an angle 0 with respect to direction ofaircraft travel, as the antenna points away from the direction oftravel, the interception of terrain by the radiated energy has anincreasing velocity differential across the beamwidth in accordance withsine 0 as is well known. This increased differential velocity causes anincrease in the frequency spread of the clutter, As the frequency bandof the clutter varies, the frequencies at which a moving target may bedetected varies, that is a moving target usually is not detectable inthe frequency bandwidth of high clutter energy.- A moving target, ofcourse, causes a different Doppler shift in reflected energy to cause areflected signal having some frequencies outside the frequenciescontaining high clutter energy.

In addition to the above described spreading of Doppler shiftedfrequencies by a scanning airborne antenna there is a change infrequency shift itself. That is, all reflected energy intercepted by theradar antenna has a change in frequency as the antenna scans from sideto side. This change in frequency shifts the band of frequencies of suchreflected energy in accordance with cosine of the angle 0. This shift iscompensated for in a coherent type of radar adjusting the localoscillator with the changing Doppler shift. In pulse type or noncoherentradar the frequency shifted signals are included in the clutterreference so as not to be detected. In any event, the absolute Dopplershift is accommodated by known techniques. It is desired to improveradar operation by maximizing the frequency bandwidth for moving targetdetection in accordance with clutter level.

In addition to the. Doppler spread referred to above, different terrainobjects (trees, water, rock, buildings, and the like) have differentradiation reflection characteristics. Therefore, as an aircraft fliesover different terrain, the level of reflected energy is subject tochange. This effect is somewhat similar to the above described Dopplerspreading insofar as moving target detection is concerned. Thisundesirable change in clutter can be accommodated when the Dopplerspreading is accommodated.

It is well known that it is desired to eliminate all clutter returnsfrom the radar processor circuits, and,

. on the other hand, it is desired to have the maximum frequencybandwidth for moving target detection. Therefore, as the radar antennain a moving vehicle scans, it is desired to adjust the clutter rejectionto accommodate the above described Doppler frequency spreading ofclutter. Prior radar systems often had a fixed range gated filterresponse designed to reject a given clutter. Such an approach may besatisfactory for a constant scan rate fixed location radar but does notmaximize moving target detection in a dynamically changing system. Otherradar units were provided with operator selection of several clutterrejection characteristics. This selection may be acceptable for a slowchanging clutter characteristic. However, in high speed .vehicles,especially those with high scan rate radar antennas, the changingclutter returns due to the Doppler effect are not adequately compensatedfor. Therefore, it is desired to have a variable clutter rejection in aradar receiver which responds to the returned signals in a manner suchas to maximize the frequency range for moving target detection aslimited by the clutter returns being received. Such an adaptive clutterrejec= tion provides an advantage to vehicle borne radar systems with ascanning antenna. Improved performance of other types of airborne andground radar systems is also provided by practicing the below describedinvention.

SUMMARY OF THE INVENTION A feature of the present invention is theutilization of plural range gated filters which are electricallyalterable by rapidly switching an electrical device therein between twoelectrical states such as to change the effective electricalcharacteristics of such device.

Another feature is the averaging of gated radar video signals within apredetermined range of interest and utilizing the average gated video toadjust the frequency characteristics of range gated filters.

Another feature is a radar system having a predetermined pulserepetitive frequency with clutter returns being typically centered atzero frequency and about multiples of the PRF wherein dynamicallyvarying band of frequencies for moving target detection are providedbetween the dynamic bounds of the clutter. Such a radar system isresponsive to changes in the clutter returns for adjusting the frequencybandwidth of the region for moving target detection.

In one embodiment of the invention, a radar transmitter and receiver issupplied main timing pulses by a radar range gate generating systemwhich also produces plural range gating signals. A plurality of rangegated filters, one filter for each range bin, receives video from theradar receiver and processes it to provide range gated video signals toa radar indicator. An adaptive averaging circuit receives the rangegated video and averages same over a predetermined range of interest andsupplies a signal having an amplitude indicative of a comparison betweenthe range gated video average amplitude and a reference amplitude. Thereference amplitude establishes a permissible average clutter amplitude.A signal responsive variable duty cycle generator receives the averagecomparison and supplies a variable duty cycle signal to the plural rangegated filters for adjusting the frequency characteristics thereof suchas to vary the frequency bandwidth of an acceptable region for movingtarget detection.

The control loop in the radar set is responsive to range gated video andsuch reference amplitude for establishing in plural range gated filtersvarying frequency characteristics in accordance with comparison of thereference amplitude and the average amplitude of the range gated video.The range gated filters may be actuated only during a predeterminedrange of interest or over the entire radar maximum range or any portionthereof.

In one version, a high pass filter of the active element type has itsfrequency characteristics changed by rapidly switching an electricallyresponsive switch connected across a component to alter the effectiveelectrical characteristics of that component. The switching rate is muchhigher than the highest frequency of signals being processed through thefilter. In one version, the electrical switch has one terminal connectedto ground reference potential and another terminal connected to an endof an electrical element remote from ground reference potential which iswithin the frequency determining portion of the filter. Such an elementmay be a resistor. In another version, it is a capacitor.

A MOSFET or any other form of field effect device is connected acrossthe feedback resistor in a filter of the active element type forswitching between current conductive and nonconductive states such as toalter the frequency characteristics thereof.

The frequency characteristics of a filter may be altered by rapidlyswitching an electrical switch between current conduction andnonconduction, i.e., between opened and closed circuits as connectedacross an electrical element forming a portion of the frequencydetermining part of a filter.

THE DRAWINGS FIG. 1 is a schematic diagram of an aircraft having anairborne radar of the scanning type to illustrate the effect of Doppleron the clutter returns in a radar system.

FIG. 2 is a graph illustrating in the frequency domain the effect ofvariation of clutter on an acceptable region for moving targetdetection.

FIG. 3 is a block type signal-flow diagram of a radar system utilizingthe present invention.

' FIG. 4 is a simplified set of idealized signal waveforms utilized todescribe the radar system illustrated in FIG. 3.

FIG. 5 is a simplified block diagram of a typical range gated filterutilizable with the FIG. 3 illustrated radar system.

FIG. 6 is a simplified schematic diagram of a highpass filter utilizablewith the FIG. 5 illustrated typical range gated filter and showingillustrative connections of various electrically responsive switchesutilized to change the frequency characteristics of the filter, i.e.,the cut off frequency.

FIG. 7 is a simplified showing of an idealized filter control signalused to control the characteristics of the FIG. 6 illustrated typicalrange gated filter.

FIG. 8 is a simplified partial schematic diagram of a switching deviceutilized to vary the effective electrical impedance of a capacitor.

FIG. 9 is a simplified block signal flow diagram of a variable dutycycle pulse generator with an illustrative and idealized ramp signalused to generate varying signal durations.

FIG. 10'is a simplified schematic block diagram of a practical adaptiveaveraging circuit utilizable in the FIG. 3 illustrated radar system.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT Referring now moreparticularly to the drawings, like numbers indicate like parts andstructural features in the various diagrams. FIG. 1 illustrates anaircraft 10 having nose radar 1 1 with a scanning antenna 12 which has aboresight beam 13, for example. As the antenna 12 scans transverse tothe direction of flight 14 of aircraft 10 to its maximum side-lookingangle 0 indicated by beam 13, the Doppler frequency changes in the beamvary as described in the Background section. If the beam 13 is displacedfrom the direction of travel 14 by an angle 0, then the Doppler spreadof clutter increases to reduce the usable bandwidth for moving targetdetection. If radar system 11 has a fixed response clutter rejector,then when the antenna 12 is scanned to the side as indicated by beam 13,some clutter returns, which are Doppler shifted, will appear in theregion for moving target detection and will then appear on the indicatorin the aircraft as moving targets.

The Doppler shifting of clutter is best understood by referring to thefrequency domain graph of FIG. 2 illustrating the response of a radarsystem in the frequency domain. The vertical ordinate represents theamplitude of the frequency components of a received signal. Thehorizontal ordinate represents the frequency of the radar beam from zerofrequency to the pulse repetitive frequency (PRF). In a pulse type radarthere are plural frequency spectrums in accordance with the Fouriertransformation of the frequencies constituting the repetitive radarpulses. Since this phenomenon is well known and all such higherfrequency spectrums are identical insofar as the present invention isconcerned, they are not shown. Ground clutter returns are normally atthe lower end of the frequency domain as indicated by the hatched area20. The horizontal width of hatched area is indicative of Doppler spreadof fixed targets (ground). Because of the characteristics of pulseradar, symmetrically identical clutter returns appearing at about theintegral multiples of the radar PRF and about the PRF are indicated bythe hatched area 21. Higher frequency clutter returns are not shown. Asthe clutter returns represented by hatched area 20 increase in frequencywidth or spread by the antenna 12 beginning to scan to the side, thereis a like increase in frequency width of hatched area 21. These twoincreases are represented by the two dotted lines 22 and 23,respectively. This characteristic of a radar set is well known and thetheory in explaining the behavior thereof will not be delved into forthat reason. This variation or Doppler spread of clutter returns aretypically at least four or five to one. That is, the movement along thefrequency ordinate of the FIG. 2 graph of the hatched areas 20 and 21varies over a distance ratio of five to one. For a homogeneous terrain,the reflected clutter energy remains constant. Therefore, the areas 20and 21 are constant. For this reasonthe height (amplitude) of theclutter is shown as reducing as frequency bandwidth is increased.Changes in terrain reflective characteristics change the reflectedenergy level which changes the areas 20 and 21.

The acceptable region for moving target detection enclosed by line 25lies between the two clutter return areas 20 and 21. As the clutterreturns increase in frequency bandwidth due to Doppler spread, theregion for moving target detection decreases as indicated by dotted line26. Line 26 corresponds to a maximum allowable region for moving targetdetection permitted by clutter represented by dotted lines 22 and 23.Correspondingly, as the frequency width or spread of the clutter return'decreases, then the region for moving target detection increases.

For optimum detection, i.e., maximizing the possibility of detecting allmoving targets, it is desired to keep the region for moving targetdetection within a radar receiver in a fixed relation to clutter returnsas shown in FIG. 2 and described above.

Therefore, a radar receiver should be responsive to clutter returns toadjust the frequency bandwidth of the region for moving targetdetection. Since, in most radar systems, there is an acceptable clutterlevel for presentation to the radar indicator, a constant reference canbe used to be compared with the average of clutter return for optimizingthe region for moving target detection with respect to such clutterreturns. To this end, the illustrated embodiment of FIG. 3 is used.

Shown in FIG. 3, radar transmitter and receiver unit 30 having antenna31 radiates interrogation or search signals in a fixed time relationshipwith main timing pulses emitted over line 32. The radiated pulse fromantenna 31 is reflected by radar targets in a known manner and theelapsed time from the time of emission to the times of return to antenna31 is a true indication of the range between the radar antenna and allradar targets. In most instances, there are a plurality of targets ataplurality of ranges. These returns are received by antenna 31 andprocessed through a receiver and supplied as radar video over line 33 toa plurality of range gated filters 34. Since the number of range gatedfilters in a given radar set may vary over a wide number, only three areshown with an ellipsis 35 indicating that any number of range gatedfilters may be utilized in practicing the present invention. Each rangegated filter processes video from line 33 to line 36 in differingpredetermined time relationships with respect to the main timing pulseson line 32; each given time relationship for the different range gatedfilters 34 are commonly referred to as range bins'. The range gatedfilter radar timing circuit 37 includes a range gate generator anddevelops the main timing pulse over line 32 and generates rangequantizing pulses 38 (FIG. 4). Although a relatively small number ofquantizing pulses are shown in FIG. 4 as occurring in the intervalbetween successive main timing pulses 39 and 40, in practice, a greaternumber of quantizing pulses 38 will normally be contained in theinterval. The purpose of FIG. 4 is to illustrate that the elapsed timebetween main timing pulses 39 and 40 is divided into a plurality of timeperiods illustrated as being betweensuccessive ones of the rangequantizing pulses 38. Each such time period corresponds to apredetermined range of the radar set which are commonly referred to asrange bins, i.e., there is one range bin between each two successivepulses 38. The elapsed time between main timing pulses 39 and 40 maycorrespond to the radar maximum range. In many applications, the maximumradar range may encompass plural timing pulses.

All the returns in a radar maximum range may not be of interest.Therefore, there is arbitrarily shown a range of interest indicatedbetween dotted lines 43 and 44. As shown in FIG. 3, the range gatedfilters 34 will only pass the radar video on line 33 existing in thetime element represented between lines 43 and 44. To this end, radartiming circuits 37 emit a plurality of range gating pulses 45 (FIG. 4)over a cable 46 which has separate signal path connections to all of therange gated filters 34 as will be described with respect to FIG. 5. Forthe purposes of FIG. 3, it suffices to say that each separate signalpath connection in cable 46 supplies a different range gating pulse 45to the respective filters. For example, line 47 carries range gatingpulse 48 (FIG. 4) to the first range gated filter 34A while line 49carries range gating pulse 50 to the second range gated filter 348. In asimilar manner, each of the range gated filters receive a range gatingpulse at different times in order to range gate the video on line 33into a different range bin. The range gating pulses 45 have a durationequal to elapsed time between two successive range quantizing pulses 33.

The range gating pulses 45 are usually generated by a counter and matrixsystem (not shown). The quantizing pulses 38 may be generated by a pulsegenerator synchronized by the main timing pulses such that receiveroperation is coordinated with the transmitter operation. Since a radartiming circuit 37 is used in every constructed radar set and can takemany variations, such circuit is not further described. It beingsufficient to say they are normally used and are utilized to generaterange bins.

The range gated video on single line 36 is received from all of thefilters 34 and supplied to an MTI (moving target indicator) or otherradar indicator, indicated by box 54, as a video signal includingreturns in all range bins.

To provide adaptive optimizing of clutter rejection, the range gatedvideo on line 36 is also supplied over line 55 to adaptive averagingcircuit 56. Adaptive averaging circuit 56 receives a reference potentialover line 57 from a source (not shown) as may be manually set by anoperator indicative of an acceptable clutter level to indicator 54.Adaptive averaging circuit 56 compares the amplitude of the range gatedvideo over a predetermined period of time, as indicated between lines 43and 44 of FIG. 4, with the reference potential and supplies a comparisonor error signal over line 58. This signal is used to adjust thefrequency bandwidth of the region for moving target detection asindicated in FIG. 2.

Variable duty cycle pulse generator 59 which generates a constantfrequency is responsive to the line 58 comparison signal to vary theduration of pulses supplied over line 60 to all range gated filters 34for adjusting the frequency characteristics thereof such that the regionfor moving target detection is adjusted in frequency bandwidth inaccordance with the comparison of the range gated video amplitude andthe reference potential on line 57. The frequency of variable duty cyclepulse generator 59 is much higher than the highest frequency processedthrough range gated filters 34. The purpose of this frequency differenceis to eliminate the introduction of spurious signals in the range gatedvideo on line 36 as could appear in indicator 54. While a variablefrequency pulse generator could be utilized to give a similar effect,the range of adjustment of the frequency response of the range gatedfilter is not as great with as simple components and configurations asusing a variable duty cycle pulse generator, as will become apparentfrom the continued reading of the present specification.

Returning now to adaptive averaging circuit 56;, the range gated videoon line 55 is supplied to a video gate 64, while the reference amplitudeon line 57 is supplied to a similar gate 65. Gates 64 and 65 receive arange enabling pulse 67 (FIG. 4) over line 66 of cable 46. Each rangeenabling pulse 67 has a duration representing the range of interestbetween lines 43 and 44 (FIG. 4). Therefore, the range gated video online 55 is utilized in averaging circuit 56 only in such range ofinterest. Optimization is to clutter in this range only. It isunderstood that optimal operation may be based on total clutter returnseven though the range gated filters 34 sample only a portion of therange. At times outside such portion integrator 70 stores the averagedcomparisons in its integrating element.

The range gated video and the reference potential are simultaneouslysupplied to the comparator 68. The comparator supplies a comparisonsignal indicative of the difference in signal amplitudes of the videoand reference potential over line 69 to integrator 70. In-

tegrator 70 then averages the comparison signals to provide the averagedvideo signal on line 58 to variable duty cycle generator 59 foradjusting the range gated filter 34 frequency characteristics inaccordance therewith.

Referring next to FIG. 5, a typical range gated filter 34 is illustratedin block diagram form. The input video is supplied first to sample andhold circuit 74 actuated by the range gating pulse 48 on line 47 for therange gated filter 34A. Sample and hold circuit 74 may be of usualdesign. The sample and held video is then supplied over line 84 throughhigh pass filter 75, thence over line 90 through low pass filter 76 andamplifier 77 to full-wave detector 78. High pass filter defines thelower frequency bound 25L (FIG. 2) and because of the pulse radarfrequency characteristics it also determines the upper frequency bound25U. It is remembered that FIG. 2 only shows the frequencies up to PRF.The detected video is supplied over line 79 to integrator 80 which inturn supplies the integrated video to an electronic switch 81. Switch 81is actuated by the range gating signal 48 on line 47. When electronicswitch 81 is actuated, the gated video is supplied to line 36 onlyduring the period of time corresponding to the range bin defined by theparticular range gated filter 34. The variable duty cycle filter controlsignal on line i 60 is supplied in the illustrated filter to high passfilter 75. The filter control signal is utilized to adjust the frequencycut off of the high pass filter 75, i.e., adjusts the lower frequencybound of the frequency pass band represented by line 25L of FIG. 2. Asthe clutter indicated by the hatched area 20 in FIG. 2 increases in thefrequency domain, the low frequency cut off of filter 75 is increased(moved to the right in FIG. 2) for reducing the region of moving targetdetection. When the clutter indicated by hatched area 20 is reduced inthe frequency domain, the low frequency cut off of high pass filter 75is decreased for increasing the bandwidth of the region of moving targetdetection.

Referring next to FIG. 6, high pass filter 75 is shown in schematicdiagram form wherein thesample and held video is received over line 84.An RC network consisting of capacitors 85 and 86 and resistors 87, 88and 91 are connected for supplying an RC filtered signal toamplifier 89.More than one such filter section may be cascaded to increase theselectivity. The filter controls can be actuated in parallel. Amplifier89 supplies its signal over line 90 to low pass filter 76 of FIG. 5. Italso supplies the output signal through feedback resistor 91 to tiepoint 92 of the RC input filter. The frequency cut off of high passfilter 75 is adjusted by rapidly switching transistor 93 between currentconduction and nonconduction. Transistor 93 is connected in parallelcircuit relation to resistor 88. The period of time of conductivity ofresistor 93 determines the reduction in the effective electricalimpedance from junction 94 to ground reference potential, and therebychanges the RC characteristics of the input circuitry to amplifier 89,hence the cut off frequency. As the transistor 93 is held to currentconduction a longer period of time the effective impedance from junction94 to ground is decreased to thereby increase the frequency cut off.This action corresponds to an increase of clutter spread in thefrequency domain. As the transistor 93 is held to current nonconductionfor longer periods of time, the effective electrical impedance fromjunction 94 to ground is increased to thereby decrease the frequency cutoff of the filter. This action corresponds to an increase in thefrequency bandwidth of the region for moving target detection, resultingfrom a decreasing clutter spread.

A filter control signal is shown in FIG. 7. It has a fixed periodbetween the leading edges of two successive pulses 100 and 101. As shownin heavy lines, the duration ofpulses 100 and 101 are short with respectto this fixed period. This corresponds to transistor 93 being currentnonconductive for a major portion of the fixed period thereby indicatingthere is a small clutter spread in the frequency domain. As the clutterin the frequency domain increases, the duration of the pulses 100 and101 are increased as indicated by dotted lines 102 to occupy greater andgreater portions of the fixed period to thereby decrease the effectiveimpedance between point 94 and ground. As the filter control signal online 60 is varied in time duration high pass filter 75 of FIG. 6 altersits cut off frequency to thereby change the region for moving targetdetection in the frequency domain.

The utilization of an electrically responsive switch, such as transistor93, connected between junction 94 and ground reference potential isdesired because the interaction of the switching to filter operation isminimized. It also simplifies circuit construction in that one terminalof the electronic switch is connected to ground reference potential. Nolimitation to such a connection is intended. It is also shown that line60 is connected to the field electrode of MOSFET 105. The source anddrain electrodes of MOSFET 105 are respectively connected to oppositeends of feedback resistor 91. As MOSFET 105 is switched between currentconduction and nonconduction, the effective electrical impedance offeedback resistor 91 is altered in the same manner that the electricalimpedance between junction 94 and ground reference potential was alteredby the switching of transistor 93. A MOSFET is desired because bothterminals of the MOSFET are connected within an active portion of thefilter. It is known that a junction transistor would pass a portion ofthe filter control signal on line 60 to the circuit, i.e., through thebase-emitter junction of transistor 93. In MOSFET 105 the fieldelectrode provides very little coupling between line 60 and its currentpath electrodes, the source and drain. This low coupling minimizes theeffect of intermodulation of the high frequency filter control signalswith the signals being processed from line 84 to line 90. Otherelectrically or optically responsive devices may be utilized as switchesfor adjusting the cut off frequency of high pass filter 75.

Not only may the resistors, such as resistors 88 and 91, have a switchconnected thereacross for altering their electrical impedance, but acapacitor may have its electrical impedance altered as shown in FIG. 8.Capacitor 85 is shown as being connected between lines 84 and 107. Asecond capacitor 108 is connected in parallel with capacitor 85. AMOSFET 109 is connected in series with capacitor 108 and can be switchedbetween current conduction and nonconduction to selectively connect anddisconnect capacitor 108 in parallel circuit relation to capacitor85 tothereby alter the capacitive impedance of the network. Such alterationobviously changes the filter characteristics of high pass filter 75.Both capacitors and 86 may have their capacitance impedance so changed.While inductances utilized in filters may be so switched, there areadditional problems because of the flyback voltages caused ininductances by rapid switching.

A circuit suitable for generating filter control signals and 101, (FIG.7) is illustrated in FIG. 9. A sawtooth generator supplies sawtooth wave111 over line 112 to differential amplifier 1 13. The averaged videosignal on line 58 is supplied to the reference input of differentialamplifier 1 13. Amplifier 1 13 supplies the instantaneous differencesignal between sawtooth wave 111 and the averaged video on line 58 overline 114 to electronic switch 115. Switch 115 is responsive to thedifference signal on line 114 of a predetermined positive amplitude tosupply a positive signal over line 60 and further responsive to suchdifference signal being less positive to supply a negative signal overline 60. Therefore, as the averaged video on line 58 increases inamplitude, there is an indication that there is an increase in clutterspread. Therefore, it is desired to raise the cut off frequency of highpass filter 75. To do this, the time constant, i.e., the electricalimpedances, must be decreased. Therefore, if the filter signal on line60 is positive only when the sawtooth waveform 111 has an amplitude lessthan the averaged video, an increase in the average video amplitude willincrease the period of time, i.e., the durations of pulses 100 and 101.This can be seen by visualizing that when line 116 drawn throughwaveform 111, is raised toward the peaks 117 that the period of timethat the sawtooth waveform 11 1 has an amplitude less than the line 116is increased thereby increasing the time that the transistor 93 (FIG. 6)is made current conductive. This action decreases the effectiveelectrical impedance from point 94 to ground reference potential toeffect the desired result for reducing the bandwidtliof the region formoving target detection. A lbwering of line 116, which represents adecrease in amplitude of the averaged video on line 58, causes acorresponding decrease in duration of the filter control signals 100 and101 to thereby increase the region for moving target detection.Inspection of FIG. 9 shows that if line 1 16 is raised above peaks 117switch 115 is in a first continuous signal state corresponding to aduration of pulses 100,101, equalling the pulse period between twosuccessive pulses, i.e., a lOO percent duty cycle. Also, if line 116 islowered below wave 111, switch 115 is held in a second signal statecorresponding to a 0 percent duty cycle.

Referring next to FIG. 10, there is shown a practical embodiment of anadaptive averaging circuit 56. The comparator 68 and the integrator 70of FIG. 3 are combined into one unit including differential amplifier120 with resistor 123 and integrating capacitor 121 connectedthereacross in a known manner, the video gate 64 output signal issupplied to the inverting input of the differential high gain amplifier120 while the gate 65 output signal is supplied to the non-invertinginput of differential amplifier 120. Differential amplifier 120 thenprovides a differential comparison of the amplitudes of the two suppliedsignals and integrates the resultant difference signals. An isolatingand inverting amplifier 122 connects the integrator-comparator to line58 for controlling variable duty cycle pulse generator 59 (FIG. 3). Whengates 64 and 65 are closed (not passing signals) capacitor 121 storesthe averaged signal amplitude until the next successive gate enablingpulse 67 is received. I

I claim:

1. A radar receiver for receiving radar signals subject to Doppleraffected clutter, including radar video processing means and timingmeans for respectively supplying a video signal and a plurality of rangegate signals,

the improvement including in combination,

a plurality of range gated filters, each range gated filter havingfrequency variable filter means responsive to a control signal forvarying electrical characteristics thereof and each supplying rangegated video in joint response to receiving a given range gated signaland said video,

an adaptive averaging circuit receiving said range gated video andcomparing same with a reference amplitude and supplying a control signalin ac cordance with such comparison to said range gated filter means foradjusting the frequency characteristics thereof such that apredetermined relationship exists between the averaged amplitudes ofsaid range gated video and said reference amplitude.

2. The subject matter of claim 1 wherein said radar timing meanssupplies a range enabling pulse indicative of a radar range of interest,

gating means in said adaptive averaging circuit receiving said rangeenabling pulse for gating said range gated video to said adaptiveaveraging circuit only during said range of interest such that the rangegated filter characteristics are affected by clutter returns only insaid range of interest.

3. The subject matter of claim 1 wherein said adaptive averaging circuitmeans further includes variable duty cycle switching means responsive tocomparison of said range gated video with a reference potential tosupply said control signal as variable duration pulses in accordancewith said comparison to all said range gated filters,

each said range gated filter including impedance switching meansresponsive to said variable duration pulses to change the effectiveelectrical impedance ofa portion of a respective one of said range gatedfilters to thereby adjust a cut off frequency therein for altering thefrequency bandwidth of such respective one of said range gated filters.

4. The subject matter of claim 3 wherein said variable duty cycleswitching means repetitively supplies said variable duration pulses at aconstant repetition rate with pulse duration being variable from zeroduration to a constant DC level corresponding to a 100 percent durationof a variable duration pulse of a given pulse period between twosuccessive ones of said variable duration pulses and said pulserepetitive rate being a much higher rate than any frequency of anysignals to be processed through any of said range gated filters.

5. The subject matter of claim 4 wherein each of said range gatedfilters includes filter means having a cut off frequency correspondingto a given clutter rejection in a frequency domain, each said filtermeans including at least one cut off frequency determining element insaid portion and exhibiting a given electrical impedance at said cut offfrequency,

said impedance switchingmeans in each said range gated filter meansbeing electrically connected across said frequency determining elementand responsive to said variable duration pulses to lower the effectiveelectrical impedance of said frequency determining element to therebyalter said given cut off frequency.

6. The subject matter of claim 1 wherein said timing means furthersupplies a range enabling pulse having a timing and duration indicativeof a range of interest,

said adaptive averaging circuit including first and second gating meansresponsive to said range enabling pulse to respectively pass said rangegated video and said reference amplitude,

differential comparison means in said adaptive averaging circuitreceiving said passed range gated video and reference amplitude andamplitude comparing same to supply a comparison signal,

integrating means in said adaptive averaging circuit receiving saidcomparison signal and averaging same during receipt of said rangeenabling pulse by said adaptive averaging circuit and holding saidaveraged comparison signal when said adaptive averaging circuit is notreceiving said range enabling pulse,

a variable duty cycle pulse generator receiving said averaged comparisonsignal and being responsive thereto to generate pulses having a constantrepetitive frequency much greater than a frequency of any signal to beprocessed by said range gated filters and varying pulse durations inaccordance with the amplitude of said averaged comparison signal,

impedance switching means in each of said range gated filters andresponsive to said constant frequency variable duration pulses to varyan ef fective electrical impedance thereof by switching between currentconduction and nonconduction states to thereby adjust electricalcharacteristics of such range gated filters in accordance with saidvariable duration pulses, said adjustment of electrical characteristicscomprising adjusting a cut off frequency such that the amplitude of saidrange gated video has a predetermined relation to said referenceamplitude whereby clutter returns in said range gated video are held toa predetermined amplitude for optimizing clutter rejection.

7. A method of clutter rejection in a moving target detecting radarsystem including comparing the amplitude of processed radar returnsignals against a reference amplitude,

adjusting the lower cut off frequency of the radar system in accordancewith such comparison such that a region for moving target detection iscontinually maximized in the frequency domain to such received clutterin such frequency domain by raising said lower cut off frequency whenthe processed radar return signal amplitudes exceed said referencedamplitude and lowering said lower cut off frequency when the amplitudeof said processed radar return signals is less than said referenceamplitude.

t t I 10! l

1. A radar receiver for receiving radar signals subject to Doppleraffected clutter, including radar video processing means and timingmeans for respectively supplying a video signal and a plurality of rangegate signals, the improvement including in combination, a plurality ofrange gated filters, each range gated filter having frequency variablefilter means responsive to a control Signal for varying electricalcharacteristics thereof and each supplying range gated video in jointresponse to receiving a given range gated signal and said video, anadaptive averaging circuit receiving said range gated video andcomparing same with a reference amplitude and supplying a control signalin accordance with such comparison to said range gated filter means foradjusting the frequency characteristics thereof such that apredetermined relationship exists between the averaged amplitudes ofsaid range gated video and said reference amplitude.
 2. The subjectmatter of claim 1 wherein said radar timing means supplies a rangeenabling pulse indicative of a radar range of interest, gating means insaid adaptive averaging circuit receiving said range enabling pulse forgating said range gated video to said adaptive averaging circuit onlyduring said range of interest such that the range gated filtercharacteristics are affected by clutter returns only in said range ofinterest.
 3. The subject matter of claim 1 wherein said adaptiveaveraging circuit means further includes variable duty cycle switchingmeans responsive to comparison of said range gated video with areference potential to supply said control signal as variable durationpulses in accordance with said comparison to all said range gatedfilters, each said range gated filter including impedance switchingmeans responsive to said variable duration pulses to change theeffective electrical impedance of a portion of a respective one of saidrange gated filters to thereby adjust a cut off frequency therein foraltering the frequency bandwidth of such respective one of said rangegated filters.
 4. The subject matter of claim 3 wherein said variableduty cycle switching means repetitively supplies said variable durationpulses at a constant repetition rate with pulse duration being variablefrom zero duration to a constant DC level corresponding to a 100 percentduration of a variable duration pulse of a given pulse period betweentwo successive ones of said variable duration pulses and said pulserepetitive rate being a much higher rate than any frequency of anysignals to be processed through any of said range gated filters.
 5. Thesubject matter of claim 4 wherein each of said range gated filtersincludes filter means having a cut off frequency corresponding to agiven clutter rejection in a frequency domain, each said filter meansincluding at least one cut off frequency determining element in saidportion and exhibiting a given electrical impedance at said cut offfrequency, said impedance switching means in each said range gatedfilter means being electrically connected across said frequencydetermining element and responsive to said variable duration pulses tolower the effective electrical impedance of said frequency determiningelement to thereby alter said given cut off frequency.
 6. The subjectmatter of claim 1 wherein said timing means further supplies a rangeenabling pulse having a timing and duration indicative of a range ofinterest, said adaptive averaging circuit including first and secondgating means responsive to said range enabling pulse to respectivelypass said range gated video and said reference amplitude, differentialcomparison means in said adaptive averaging circuit receiving saidpassed range gated video and reference amplitude and amplitude comparingsame to supply a comparison signal, integrating means in said adaptiveaveraging circuit receiving said comparison signal and averaging sameduring receipt of said range enabling pulse by said adaptive averagingcircuit and holding said averaged comparison signal when said adaptiveaveraging circuit is not receiving said range enabling pulse, a variableduty cycle pulse generator receiving said averaged comparison signal andbeing responsive thereto to generate pulses having a constant repetitivefrequency much greater than a frequency of any signal to be processed bysaid rAnge gated filters and varying pulse durations in accordance withthe amplitude of said averaged comparison signal, impedance switchingmeans in each of said range gated filters and responsive to saidconstant frequency variable duration pulses to vary an effectiveelectrical impedance thereof by switching between current conduction andnonconduction states to thereby adjust electrical characteristics ofsuch range gated filters in accordance with said variable durationpulses, said adjustment of electrical characteristics comprisingadjusting a cut off frequency such that the amplitude of said rangegated video has a predetermined relation to said reference amplitudewhereby clutter returns in said range gated video are held to apredetermined amplitude for optimizing clutter rejection.
 7. A method ofclutter rejection in a moving target detecting radar system includingcomparing the amplitude of processed radar return signals against areference amplitude, adjusting the lower cut off frequency of the radarsystem in accordance with such comparison such that a region for movingtarget detection is continually maximized in the frequency domain tosuch received clutter in such frequency domain by raising said lower cutoff frequency when the processed radar return signal amplitudes exceedsaid referenced amplitude and lowering said lower cut off frequency whenthe amplitude of said processed radar return signals is less than saidreference amplitude.